Align eges to remove holes when using mirror modifier
There are few things you can do.


Enable clipping in mirror modifier options, select all and move everything towards middle point until top and bottom edges will align.
Rotate the object on the Y axis to align all edges.
Select top and bottom edges and press S then X and 0 to align them on X axis.
With clipping enabled, move all closer to the middle until closer edge will stack to it's counterpart on the other side, select further edge and with proportional editing enabled, pull it closer to the middle point.